KAIST team boosts heat-transfer efficiency 5.5-fold by controlling how droplets form and fall

Making water droplets form more readily and fall away faster has allowed researchers to boost heat-transfer performance by up to 5.5 times.

KAIST announced Sunday that a joint research team led by Nam Young-seok, a professor in the Department of Mechanical Engineering, and Lim Sung-gap, a professor in the Department of Chemical and Biomolecular Engineering, has developed a technology that controls the thickness and structure of an ultra-thin polymer coating applied to a surface — causing more droplets to form when water vapor condenses and making those droplets fall away more quickly.

The condensation of water vapor into liquid is a familiar phenomenon — the same process that causes droplets to bead on the outside of a cold drink. In industrial settings, it is widely used to convert steam back into water at power plants, to extract fresh water from seawater, and to dissipate heat generated by electronic devices.

Existing technologies, however, faced a fundamental dilemma. Roughening a surface to create more nucleation sites — places where droplets first form — caused droplets to become trapped in the surface structure and resist falling off. Smoothing the surface had the opposite effect: droplets shed easily, but fewer nucleation sites were available for new ones to form.

The research team solved this problem by exploiting nanoscale particles, known as aggregates, that had previously been regarded as defects in polymer films. A polymer film is an extremely thin coating of a plastic-like polymer material applied to a surface.

The team used initiated chemical vapor deposition (iCVD), a technique that deposits gaseous precursors onto a surface to form an ultra-thin polymer film. When the film was made thin enough, dense clusters of nanoscale particles appeared on the surface and acted as nucleation sites where droplets first began to form. As a result, the thin polymer film produced roughly three times as many droplets as a thicker film.

The team then applied heat treatment to reduce the adhesion force holding droplets to the surface, allowing them to detach before growing too large. Thinning the polymer film increased the number of nucleation sites, while heat treatment caused the droplets that formed to shed quickly. The core achievement of the research was solving both challenges — generating more droplets and removing them faster — simultaneously.

The team applied the polymer coating to copper tubes — widely used in condensers — and measured the results. The condensation heat-transfer coefficient, which indicates how effectively heat is transferred, reached a maximum of approximately 88 kW·m⁻²·K⁻¹, up to about 5.5 times higher than that of an untreated copper surface on which a continuous water film forms.

Unlike conventional approaches that focused on smoothing surfaces or maximizing water repellency, this research actively exploited surface "defects." The team found that the nanoscale particles previously considered impurities in fact help droplets nucleate more readily, and incorporated that insight into a new surface design strategy.

The technology could eventually be applied to power-plant steam condensers and industrial heat exchangers to improve energy efficiency. It is also expected to enhance water collection in desalination and atmospheric water-harvesting systems, and to accelerate heat dissipation for better cooling of electronic devices.

"This research is significant in that it turned nanostructures previously regarded as defects into features that promote droplet nucleation," Nam said. "We have proposed a new method of improving heat-transfer efficiency by independently controlling droplet formation and removal."

He added that further validation would be needed before industrial application, including long-term durability testing and the development of processes to coat large heat exchangers uniformly.

The findings were published online in the journal Nature Communications on July 16.